This invention is in the field of stents for implantation into a vessel of a human body.
Stents are well known medical devices that have been used for maintaining the patency of a large variety of vessels of the human body. The most frequent use is for implantation into the coronary vasculature. Although stents have been used for this purpose for more than ten years, many stent designs still lack the required flexibility and radial rigidity to provide an optimum clinical result. Another deficiency of open cell stents is that some stent struts members can flare outward (fish scaling) as the stent is advanced through a tight curve.
Most current tubular stents use a multiplicity of circumferential sets of strut members connected by either straight longitudinal connecting links or undulating longitudinal connecting links. The circumferential sets of strut members are typically formed from connecting diagonal and curved sections forming a closed ring that opens up as the stent expands to form the structural element in the stent that pushes against the arterial wall. A single strut member is defined as adjacent connected diagonal and curved sections within one of the circumferential sets of strut members.
An open cell stent is defined as a stent that has circumferential sets of strut members with most of the curved sections (crowns) that are not connected by a longitudinal connecting link to an adjacent circumferential set of strut members. In comparison, a closed cell stent has every curved section of every circumferential set of strut members, except at the distal and proximal ends of the stent, attached to a longitudinal connecting link. A strut member whose curved section is not attached to a longitudinal connecting link is defined as an unconnected strut member.
There are several xe2x80x9copen cellxe2x80x9d stents that are currently being marketed for the treatment of coronary stenoses. Examples of these are the Tetra stent from Guidant Corporation and the S670 stent from Medtronics, Inc. Each of these stents has a limited number of straight longitudinal connecting links to join adjacent curved sections of adjacent circumferential sets of strut members. These straight longitudinal connecting links can cause outward flaring of the end circumferential sets of strut members as the stent is bent around a curve. The interior unconnected strut members also can flare outward when the pre-deployed stent mounted on a balloon is advanced through a curved vessel such as a coronary artery. Any flared out strut can engage the vessel wall during stent delivery in a curved vessel thereby preventing the stent from reaching the site that is to be stented.
The present invention is a stent that is designed to optimize many of the operating parameters that are expected for stents in the first decade of the 21st century. Specifically, an optimum design would have the following characteristics:
I. IN THE PRE-DEPLOYED STATE
1. excellent flexibility
2. low profile (i.e.; small outside diameter of the stent)
3. good radiopacity
4. smooth outer surface
5. no flaring of struts when advancing through curved arteries
6. a high degree of stent retention onto the delivery catheter
II. AFTER DEPLOYMENT
1. flexible so as to conform to a curved artery
2. radially rigid (i.e.; low recoil)
3. good radiopacity
4. good coverage of the vessel wall (i.e.; no plaque prolapse)
5. side branch access without strut breakage
6. minimal foreshortening compared to the length of the stent in its pre-deployed state
Although many desirable attributes are required of the catheter that is used to deliver the stent, the scope of the present invention is limited to the design of the stent itself. However, it should be understood that the reduced foreshortening of this stent is a result of having undulating longitudinal connecting links that easily extend in their longitudinal length when the balloon onto which the stent is crimped is inflated.
To accomplish the goals listed in I. and II. above, the stent would optimally have at least two open cells around the circumference of the stent. A unique feature of the present invention is that each of the strut members whose curved sections are unconnected has a shorter longitudinal length as compared to the longitudinal length of the strut members that are connected by a longitudinal connecting link. This shorter length (optimally on the order of 0.1 mm shorter) reduces outward flaring of the unconnected strut members when the stent is advanced through highly curved vessels such as some coronary arteries. Flaring (which is sometimes called xe2x80x9cfish-scalingxe2x80x9d) can cause the stent to engage the vessel wall as the stent is advanced through curved arteries.
Another novel feature of this stent is that the longitudinal connecting links can have an undulating shape so that they can easily expand or contract in their longitudinal length when the stent is advanced through a curved vessel. The extraordinary capability of this stent to bend easily is a combination of the fact that those curved sections of adjacent circumferential sets of strut members that are connected are connected with flexible longitudinal connecting links, and many (typically one-half) of the curved sections are unconnected. Of course, the weakest possible connection that provides the highest degree of longitudinal flexibility is no connection at all. Therefore, the combination of no connections plus the few required connections between the circumferential sets of strut members being by means of highly flexible undulating longitudinal connecting links imparts to this stent an extraordinarily high degree of longitudinal flexibility.
It should also be understood that all the strut members at each end of the stent should also have a shortened longitudinal length because the outside curved section of the end circumferential sets of strut members cannot be connected to any adjacent circumferential set of strut members. By shortening all the end strut members, end flaring of the stent as it is advanced through curved vessels can be reduced. Furthermore, the fact that the interior curved sections of each strut member at the ends of the stent either has either no connection or a flexible, undulating longitudinal connecting link connection to an inner strut members, is also desirable in preventing flaring out of the strut members at the ends of the stent. This is not the case for strut members that have a straight connection to an end circumferential strut such as shown in FIG. 5 of U.S. Pat. No. 5,759,192.
Good radiopacity for the stent is achieved by having a stainless steel stent that has a wall thickness that is at least 0.0045 inches. Another means would be to use a metal with a higher density such as tantalum with a thickness greater than 0.002 inches. A third means for obtaining improved radiopacity would be to sandwich a high density metal between two layers of stainless steel with each of the co-axial tubes having a wall thickness between 0.001 and 0.002 inches with the total wall thickness of the stent being at least 0.003 inches.
Another feature of the present invention is that the undulating longitudinal connecting links readily extend in the longitudinal direction when the balloon is inflated. Since the circumferential sets of strut members upon deployment tend to decrease in their longitudinal length, the longitudinal lengthening of the undulating longitudinal connecting links has the effect of minimizing the foreshortening of the deployed stent.
Thus an object of the present invention is to have increased longitudinal flexibility for the stent by having some curved sections of each circumferential set of strut members being unconnected to the curved sections of the adjacent circumferential set of strut members with the other curved sections being connected by highly flexible, undulating longitudinal connecting links.
Another object of the present invention is to prevent flaring of the unconnected strut members by having a shorter longitudinal length for the unconnected strut members.
Still another object of the invention is to have generally shorter longitudinal lengths for all the strut members in the circumferential sets of strut members at the stent ends to reduce the tendency for end flaring of the stent.
Still another object of the invention is to decrease the propensity of the stent to have end flaring by having each curved section of the end set of strut members either being unconnected to the curved section of the adjacent (interior) set of strut members or being connected by a highly flexible undulating longitudinal connecting link.
Still another object of the invention is to connect the flexible longitudinal connecting links to the curved section of the connected strut members at a point at or near the point where the curved section of each connected strut members is joined to the diagonal section of that connected strut member, thus further reducing the propensity for end flaring.
Still another object of the invention is to have flexible longitudinal connecting links that are adapted to readily increase their longitudinal length when the balloon is inflated; thus minimizing the extent of stent foreshortening upon stent deployment.
These and other objects and advantages of this invention will become obvious to a person of ordinary skill in this art upon reading the detailed description of this invention including the associated drawings as presented herein.